This invention relates to an optical atmospheric link apparatus which transmits information data through a light beam, and more particularly which automatically corrects the projection direction of the light beam. Conventionally, an optical atmospheric link apparatus has been proposed (Japanese Patent Application No. 2-276328) which observes a reflected portion of a light beam sent to a transmission object, together with an observation light coming from the transmission object, so as to readily confirm the correct illuminating position of the light beam.
That is, as shown in FIG. 1, an optical atmospheric link apparatus 1 drives a laser diode 2 by means of a predetermined information signal to project a light beam LA1 having a predetermined polarization plane from the laser diode 2. A lens 4 directs the light beam LA1 to a half-silvered mirror 8 through a polarizer 6 after having converted it to a parallel light beam. The half-silvered mirror 8 transmits a portion of the light beam LA1 therethrough and sends the transmitted light beam to the transmission object through lenses 16, 18. In this way the optical atmospheric link apparatus 1 can send the light beam LA1 having the predetermined polarization plane to the transmission object.
A reflection of the light beam LA1 is reflected by means of a corner cube prism 10, and the half-silvered mirror 8 directs the reflected light beam to an image pickup element 14 through a lens 12. Thereby, the optical atmospheric link apparatus 1 can separate a portion of the light beam LA1 sent to the transmission object and condense it to the image pickup element 14 after its reflection along its optical path.
The lens 18 also receives a light beam LA2 coming from the transmission object and directs it to the polarizer 6 through the lens 16 and the mirror 8. The transmission object projects the light beam LA2 so as to have a polarization plane which perpendicularly intersects the polarization plane of the light beam LA1 so that after the light beam LA2 passes through the half silvered mirror 8, is reflected by the polarizer 6, the optical atmospheric link apparatus 1 condenses the light beam LA2 onto a light receiving element 22 through a lens 20. Thus, the optical atmospheric link apparatus 1 receives the information by receiving the light beam LA2 coming from the transmission object.
Further, the lens 18 receives a light L1 (referred to as an observation light hereinafter) advancing from a scene around the transmission object to the optical atmospheric link apparatus 1 together with the light beam LA2 and directs the observation light L1 to the image pickup element 14 through the lens 16, the half-silvered mirror 8 and the lens 12. A component of the observation light L1, which component has an optical axis parallel to the light beam LA1, enters the lens 12 in parallel to the light reflected from the corner cube prism 10. Accordingly, the reflected light from the corner cube prism 10 enters the lens 12 along an optical path as if it was projected from the illuminating position of the light beam LA1 toward the image pickup element 14. The optical atmospheric link apparatus 1 provides a picked-up video having a light bright-point formed at the illuminating position of the light beam LA1, through the image pickup element 14, and confirms readily the illuminating position of the light beam LA1.
A pair of the optical atmospheric link apparatuses can be provided opposite to each other at two desired positions for use as a transmitting means on a relay of TV program. However, in the conventional optical atmospheric link apparatus which is used as the transmitting means, if either of the optical atmospheric link apparatus are shaken, the direction of the projecting light beam from one of the optical atmospheric link apparatuses might get out of alignment to receive the light beam from the other optical atmospheric link apparatus, as for example a truck runs on a road near the optical atmospheric link apparatuses, or the wind blows around the optical atmospheric link apparatuses.